The science magazine of Imperial College

‘Iron rain’ fell on early Earth

It has previously been theorised that most iron-rich objects from space that collided with early Earth melted into its core. This model was based on a prediction of how iron would behave in the extreme conditions that iron-based meteors experienced in their collisions with Earth.

A team of scientists at the Sandia National Laboratory in New Mexico determined the actual behaviour of iron under such conditions using simulations. Due to the high-energy of these iron collisions, it was not possible to simulate the true early Earth system conditions.

As Dr. Richard Kraus of the team notes, “We’re never really going to be able to get a situation where we can simulate the actual planetary impact, with objects a thousand kilometres across. It would just be too destructive. We’re taking a step back and saying, let’s make a fundamental measure of the entropy of iron.”

The team used Sandia National Laboratory’s Z machine to accelerate iron samples to high speeds to simulate early Earth’s extreme pressure and temperature. The iron samples were fired at aluminium plates, whose collisions were powerful enough to turn the iron to vapour.

This experimental technique has been lauded by many throughout the scientific community. According to geology and geochemistry Professor James Day at the University of California San Diego, this method was “… a really ingenious way of looking at this problem. I think this paper will be very important for future studies of the geochemistry of Earth.”

Dr. Krauss and his team recently published their findings, including the discovery that iron vapourises at a pressure 40% lower than originally thought. This discovery implies that iron-rich meteors that hit early Earth vapourised upon impact. This impact vapourisation would have dispersed iron and rock over the surface of the Earth, which would later precipitate out in the form of iron rain.

This new iron rain theory explains not only why metals can be found in Earth’s mantle and crust, but also why similar metals are not found with the same abundance on the surface of the moon. The meteor collisions occurred on both early Earth and the moon, but Earth’s larger gravitational pull was able to retain the vapourised iron in its atmosphere while the moon’s smaller pull could not prevent the molecules from escaping. Therefore, the iron could precipitate out on Earth, but not on the moon, which explains the different compositions in the Earth and moon surfaces.